There’s one Earth system that constantly affects our lives, yet we hardly think about it. That is, until we turn 50 and we’re confronted with the evidence. I’m talking, of course, about gravity.

Working against it keeps our muscles toned, and the rest of us…not so toned. But it keeps us standing upright, keeps water in our glasses and in the oceans. In fact, Earth as we know it couldn’t exist without gravity.

All objects exert a gravitational pull on other objects. The strength of this pull is determined by the object’s size and density.

The sun has more gravitational pull than Earth, which holds Earth in orbit around it. The moon has less than Earth—and thus orbits around us—but still has enough gravity to pull on Earth’s water, causing tides.

If Earth had a smooth surface, and were the same density in all places, gravity would be the same everywhere. But it’s neither of these things.

If you’re standing next to a mountain and holding a lead weight on a string, that weight would not hang perfectly straight but would be drawn slightly toward the mountain. Not enough for you to notice, but measurable with a sensitive instrument.

Mountain ranges in general have stronger gravitational pull than, say, oceans, since rock is denser than water. It’s the different densities of different places on Earth that cause Earth’s gravity to vary.

We’ll explore the surprising things that changing gravity reveals on a future EarthDate.